Once you have Kindle Unlimited, you can PURCHASE the Audible Audiobook versions at a reduced price. This will let you keep them even after you have cancelled your Kindle Unlimited package.
Follow these steps:
-Add the Kindle book to your unlimited library (make sure it says "Read for Free", DONT CLICK BUY)
-Click on "My Kindle Unlimited Library" or go to this page[amazon.com] and DELETE the audiobook from your library (may take a minute to show up)
-Go back to the kindle page and switch the format from kindle to audiobook, and you can purchase it for the reduced price
-Return the kindle unlimited book to make space in your library for more